162
C. C. V. Velloso et al.
On the other hand, plants can select endophytic bacteria through steps that function
as filters primarily in the rhizosphere, then rhizoplane, and endosphere. The rhizosphere is described as the main compartment or “gateway” that deeply influences
the plant’s endophytic microbiome. The different physicochemical and biological
properties of the carbon-rich molecules and antimicrobial compounds exuded in this
region may favor the growth and multiplication of certain groups of microorganisms
while inhibit others. Rhizoplane functions as a second selection point for microorganisms, where only those capable of binding to the root surface are allowed to enter
the endosphere (Edwards et al. 2015; Reinhold-Hurek et al. 2015). On the other
hand, the plant’s immune system actively excludes specific groups of microorganisms
(Lundberg et al. 2012).
Root colonization often begins with chemotaxis, i.e., bacterial recognition of
certain compounds in root exudates. Although there is no direct evidence of the
presence of a specific compound, flavonoids are considered as an important player
in plant-microorganism communication (Shaw et al. 2006). Once in the rhizosphere,
endophytic bacteria must bind to the root surface (rhizoplane) to reach entry sites
such as lateral emergence and root tips or regions with cracks caused by pathogens or
predators. Bacterial traits such as motility, polysaccharide production, and adhesins
are important in the root surface adhesion process (Hori and Matsumoto 2010). As
they bind to the root surface, bacteria multiply resulting in the establishment of microcolonies or biofilms thus, become successful in the colonization process. The host
penetration process can be passive or active. In passive, the bacterium uses fissures
already present in the root and in the active, penetration occurs through the production
of lipopolysaccharides, flagella, pili, and quorum sensing (Böhm et al. 2007; SuárezMoreno et al. 2010). In the active process, secretion of cell wall degrading enzymes
such as pectinases and cellulases are described as important mechanisms for the
penetration and colonization of bacteria within host plants (Compant et al. 2005).
As endophytic bacteria enter the plant, they respond to host stimuli to induce
the cellular processes necessary for the maintenance of the endophytic stage and
distribution to cortical tissue of the root. At this point, they can multiply within
the tissues often reaching high populations, depending on the stage of plant development (Hardoim et al. 2008). Migration of root bacteria to shoot tissues requires
the production of cell wall degrading enzymes. It can also occur through xylem
elements, directed by plant transpiration, which allow the movement of bacteria
mainly reaching the leaf tissues. Only few bacteria can migrate and adapt to the
shoot, as this colonization requires specific physiological signals to occupy plant
niche (Hallmann 2001).
In general, endophytic microbial communities are less diverse than the rhizospheric communities, for both bacteria and fungi. This suggests that roots select
the endosphere community members that consequently present more defined groups
(Bulgarelli et al. 2013). The bacterial endophyte community patterns in leaf, stem, and
root of three tropical rainforest plant species show a lower diversity of OTU richness,
species richness, and community diversity (inversed Simpson’s index) in comparison to that of rhizospheric soil community (Haruna et al. 2017). This emphasizes
that, the bacteria rhizospheric community from these three plant species is relatively
C. C. V. Velloso et al.
On the other hand, plants can select endophytic bacteria through steps that function
as filters primarily in the rhizosphere, then rhizoplane, and endosphere. The rhizosphere is described as the main compartment or “gateway” that deeply influences
the plant’s endophytic microbiome. The different physicochemical and biological
properties of the carbon-rich molecules and antimicrobial compounds exuded in this
region may favor the growth and multiplication of certain groups of microorganisms
while inhibit others. Rhizoplane functions as a second selection point for microorganisms, where only those capable of binding to the root surface are allowed to enter
the endosphere (Edwards et al. 2015; Reinhold-Hurek et al. 2015). On the other
hand, the plant’s immune system actively excludes specific groups of microorganisms
(Lundberg et al. 2012).
Root colonization often begins with chemotaxis, i.e., bacterial recognition of
certain compounds in root exudates. Although there is no direct evidence of the
presence of a specific compound, flavonoids are considered as an important player
in plant-microorganism communication (Shaw et al. 2006). Once in the rhizosphere,
endophytic bacteria must bind to the root surface (rhizoplane) to reach entry sites
such as lateral emergence and root tips or regions with cracks caused by pathogens or
predators. Bacterial traits such as motility, polysaccharide production, and adhesins
are important in the root surface adhesion process (Hori and Matsumoto 2010). As
they bind to the root surface, bacteria multiply resulting in the establishment of microcolonies or biofilms thus, become successful in the colonization process. The host
penetration process can be passive or active. In passive, the bacterium uses fissures
already present in the root and in the active, penetration occurs through the production
of lipopolysaccharides, flagella, pili, and quorum sensing (Böhm et al. 2007; SuárezMoreno et al. 2010). In the active process, secretion of cell wall degrading enzymes
such as pectinases and cellulases are described as important mechanisms for the
penetration and colonization of bacteria within host plants (Compant et al. 2005).
As endophytic bacteria enter the plant, they respond to host stimuli to induce
the cellular processes necessary for the maintenance of the endophytic stage and
distribution to cortical tissue of the root. At this point, they can multiply within
the tissues often reaching high populations, depending on the stage of plant development (Hardoim et al. 2008). Migration of root bacteria to shoot tissues requires
the production of cell wall degrading enzymes. It can also occur through xylem
elements, directed by plant transpiration, which allow the movement of bacteria
mainly reaching the leaf tissues. Only few bacteria can migrate and adapt to the
shoot, as this colonization requires specific physiological signals to occupy plant
niche (Hallmann 2001).
In general, endophytic microbial communities are less diverse than the rhizospheric communities, for both bacteria and fungi. This suggests that roots select
the endosphere community members that consequently present more defined groups
(Bulgarelli et al. 2013). The bacterial endophyte community patterns in leaf, stem, and
root of three tropical rainforest plant species show a lower diversity of OTU richness,
species richness, and community diversity (inversed Simpson’s index) in comparison to that of rhizospheric soil community (Haruna et al. 2017). This emphasizes
that, the bacteria rhizospheric community from these three plant species is relatively